Microstructure of modern sea pen axes – a tool for the systematics of fossil pennatulaceans (Octocorallia)

Dr Mike Reich, University of Göttingen

Thursday 29th November

Neil Chalmers Seminar Room, DC2, 12:00 (the first of two talks that day)

Pennatulaceans are considered to be a very distinct and specialised group of octocorallian cnidarians, known from soft bottom areas of the intertidal to the deep sea. Since they are largely composed of soft tissue, the systematics of modern sea pens (Anthozoa: Octocorallia) are mostly based on soft-part morphology. Among the only hard parts (an axial rod and various sclerites) present in most species, only the sclerites have been employed as systematic traits. However, both types of hard parts, especially the calcareous axial rod, are often the only remnants of fossil Pennatulacea. Fossil sea pen axes (known since the Late Cretaceous) have therefore mostly been placed in the ‘collective genus’ Graphularia without any further systematic assignment. A study of hard-part morphology, using 20 modern sea pen species from 10 of the 14 valid families, has shown that the microstructure of the axial rod is relatively consistent within a genus, whereas the cross-section may vary between the distal ends of the rachis and the peduncle. Using field emission SEM, the microstructure of at least a dozen fossil Graphularia species have been studied. By comparing the microstructure of modern and fossil sea pens, similarities allow a placement of fossil pennatulacean species close to modern genera and within modern families, demonstrating the value of axis microstructure as a systematic trait.

Today, biodiversity increases from polar to equatorial regions. Despite representing a fundamental pattern governing the distribution of the vast majority of life on Earth today, the causes of this latitudinal biodiversity gradient remain unresolved. Understanding this pattern is critical to predictions of climatically-driven biodiversity loss. The fossil record offers a unique perspective on the evolution of this gradient, providing a dynamic system in which to explore spatiotemporal diversity fluctuations. Deep time studies indicate that a gradient was present from 500 million years ago, but that it has not been a persistent pattern throughout the history of complex life. Instead, recent work has revealed a palaeotemperate peak in Mesozoic dinosaurs and other early Cenozoic groups, suggesting that the steep, modern-type gradient might only have come into existence in the last 30 million years